1. Field of the Invention
This invention relates to methods for characterizing surfaces of catalytic materials by using positron annihilation radiation lineshapes. More specifically, it relates to the detection of changes in surface area, surface acidity, and specific surface acidity of zeolite catalysts by using positron annihilation radiation lineshapes produced by a single high resolution gamma detector or by two-dimensional angular correlation measurements. In this particular field of surface characterization of zeolite catalysts, the emphasis is upon extending the positron annihilation technique to the achievement of an in-situ, efficient, and non-destructive method for the measurements of surface area and acidity.
2. Background Information
Zeolite catalysts with large internal surface areas (in hundreds m.sup.2 /gm) play a vital role in certain sectors of the chemical industry. For example, catalysts of zeolites incorporated in silica, silica-alumina, and silica-clay materials can drastically improve the yield of petroleum-cracking process, resulting in saving the refineries billions of dollars every year. Zeolite ZSM-5, as another example, was found to be invaluable in some large-scale processes, such as M-forming (increasing octane number in gasoline), dewaxing, high yield ethylbenzene production and methanol (from coal or natural gas) conversion to high grade gasoline, etc.
The catalytic process is rather complicated. It primarily involves such factors as diffusion and adsorption of reactants on the catalyst surface, desorption and diffusion of products away from the surface, etc. It is apparent, therefore, that the size, shape, and chemical characteristics of the internal surfaces are crucially related to the activity and performance of the catalyst. In the face of increasing demand for improvements in air quality as required by the environmental regulations and improvement in fuel efficiency, the need to understand and to search better ways to characterize zeolite catalysts has become ever more urgent and important.
Conventional methods of acidity evaluation include UV-visible spectroscopy (Hammett-type measurements), study of thermal deposition of salts, measurement of heat of interaction with bases, and measurement of rates of model reactions.
The Hammett-type measurements are not useable for practical purposes, because real life catalysts are normally colored and opaque. The thermogravimetric and calorimetric techniques have not been able to distinguish the effects of protonation of the base from the effects of other types of interaction of the base with the catalyst. The method based on reaction kinetics requires an intimate knowledge of the reaction mechanism itself which is often based on postulations. Measurement of the protonation by C-13 NMR method has a low sensitivity and requires the use of concentration of base whose cancellation of the activity term is not usually available (Reference 7).
Other traditional methods for the determination of surface properties of zeolites, for example, the chemisorption method for measuring surface acidity of zeolites, often yield inconsistent results among different laboratories, and at times fail to provide a valid assessment on the activity of the zeolite.
Positron annihilation is an unconventional, in-situ microprobe for monitoring electronic properties of the sample. Its principle is based on the fact that positrons, the positively charged anti-particle of electrons, when impregnated in the sample, always annihilate with electrons in the sample material, resulting in gamma rays according to the famous energy-matter equation E=mc.sup.2 which is 511 KeV for an electron. The method includes three techniques, namely, positron lifetime, angular correlation (one-, and two-dimensional), and annihilation radiation lineshape measurements. Prior art in the field almost exclusively employed positron lifetime technique for the characterization of catalytic and porous materials.
In order to provide background information so that the invention may be completely understood and appreciated in its proper context, reference is made to a number of prior publications as follows:
Reference 1, authored by J. Lahtinen and A. Vehanen, provides an overview of the positron techniques as applied to surface studies. It extensively reviewed in particular the application of positron lifetime method.
Reference 2, authored by Y. Ito and T. Takano, presented positron lifetime and one-dimensional angular correlation data for a series of synthetic zeolites. Results were correlated to the size of voids, not surface properties, of materials.
Reference 3, authored by K. Venkateswaran, K. L. Cheng, and Y. C. Jean, illustrated the application of positron lifetime technique to porous resins. A correlation between the intensity of the long-lived ortho-positronium and the effective surface area was shown.
Reference 4, authored by M. B. Perkal and W. B. walters, reported results of studies of zeolites 4A and 13X by positron technique, still by positron lifetime measurements. The results were considered to be related to pore sizes.
Reference 5, authored by H. Nakanishi and Y. Ujihira, is pertinent to showing results of application of the positron techniques to the characterization of some zeolites, primarily by positron lifetime and, to a much lesser extent, by Doppler-broadening measurements. Its purpose was to study the character of zeolite cages.
Reference 6, authored by W. F. Huang, R. Ochoa, and R. Miranda, uses Doppler broadened positron annihilation spectra in silica-alumina to demonstrate that positroniums formed in this material preferentially interact with Bronsted acid sites, not with Lewis acid sites.
Reference 7, authored by D. Farcasiu, G. Miller, A. Ghenciu, and H. S. Cao, discusses deficiencies of various conventional methods of determining the acidity of catalysts and presents a modification to the technique of C-13 NMR measurements.
Positron lifetime spectrum, as employed by previous investigators, has complex structures in molecular substances. It can have two, three, or four components which superimpose on each other, forming a continuous distribution. The number of components to be used for analysis is arbitrarily decided. And the specific lifetime component chosen to correlate with material's characteristics varies among investigators. The intensity of any particular time component often lacks the consistency that may enable interpretation of results.
Positron lifetime technique is inherently a time-inefficient method. Because of the requirement of low counting rate for positron lifetime measurement, a single lifetime spectrum usually takes several days. One-dimensional angular correlation of the annihilation radiation is also a very time consuming process due to the extremely fine opening required for the parallel slits system used in this technique. All the useful annihilation radiation outside the fine slits are not utilized. It is, therefore, neither a positron source effective, nor a cost effective technique. On the other, using the half-width of the narrow component of the one-dimensional angular correlation spectrum has not shown any correlation with the internal surface parameters.
Whatever the precise merits, features and advantages of the above cited references, none of them achieves or fulfill the purposes of surface characterization of catalytic materials by using positron annihilation radiation lineshapes in the present invention.
It is therefore the principal object of the present invention to provide an efficient, in-situ method which can directly provide information about surface characteristics of catalytic materials.